scholarly journals Inhomogeneous equation of state of the universe: Phantom era, future singularity, and crossing the phantom barrier

2005 ◽  
Vol 72 (2) ◽  
Author(s):  
Shin’ichi Nojiri ◽  
Sergei D. Odintsov
Keyword(s):  
Equation Of State ◽  
Inhomogeneous Equation ◽  
Future Singularity ◽  
Phantom Barrier ◽  
The Universe

FRW viscous cosmology with inhomogeneous equation of state and future singularity

2015 ◽  
Vol 30 (29) ◽  
pp. 1550144 ◽  
Author(s):  
G. S. Khadekar ◽  
Deepti Raut ◽  
V. G. Miskin
Keyword(s):  
Equation Of State ◽  
Bulk Viscosity ◽  
Dynamical Equation ◽  
Cosmological Evolution ◽  
Time Dependent ◽  
Inhomogeneous Equation ◽  
Future Singularity ◽  
Big Rip ◽  
Dependent Parameter ◽  
The Universe

A universe media is considered as a bulk viscosity described by inhomogeneous equation of state (EOS) of the form [Formula: see text], where [Formula: see text] is a time-dependent parameter. A generalized dynamical equation for the scale factor of the universe is proposed to describe the cosmological evolution, in which we assume the bulk viscosity and time-dependent parameter [Formula: see text] are linear combination of two terms of the form: [Formula: see text] and [Formula: see text], i.e.[Formula: see text]one is constant and other is proportional to Hubble parameter [Formula: see text]. In this framework, we demonstrate that this model can be used to explain the dark energy dominated universe, and the inhomogeneous term of specific form introduced in EOS, may lead to three kinds of fates of cosmological evolution: no future singularity, big rip or Type[Formula: see text]III singularity as presented by [S. Nojiri and S. D. Odintsov, Phys. Rev. D 72, 023003 (2005)].

Cosmology of viscous holographic f(G) gravity and consequences in the framework of quintessence scalar field

2019 ◽  
Vol 16 (11) ◽  
pp. 1950176
Author(s):  
Swati Sinha ◽  
Surajit Chattopadhyay ◽  
Irina Radinschi
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Equation Of State ◽  
Holographic Dark Energy ◽  
Inhomogeneous Equation ◽  
Late Time ◽  
Effective Equation ◽  
Future Singularity ◽  
Warm Inflation ◽  
The Universe

Work reported in this study demonstrates the reconstruction schemes for the [Formula: see text] gravity in the framework of bulk viscosity and holographic background evolution by considering the universe filled by the viscous fluid that is just special class of more general fluids as described in Nojiri and Odintsov [Inhomogeneous equation of state of the universe: Phantom era, future singularity, and crossing the phantom barrier, Phys. Rev. D 72 (2005) 023003]. The bulk viscous pressure has been considered as [Formula: see text], with [Formula: see text]. Considering the scale factor in power law form and taking holographic dark energy (HDE) with density [Formula: see text] and generalized extended holographic dark energy (EGHRDE) with density [Formula: see text], a specific case of Nojiri–Odintsov holographic DE ([Unifying phantom inflation with late-time acceleration: Scalar phantom–non-phantom transition model and generalized holographic dark energy, Gen. Relativ. Gravit. 38 (2006) 1285]) we have derived solutions for [Formula: see text] and the subsequent effective equation of state parameters have been found to behave like quintom irrespective of the choice of [Formula: see text]. Finally, considering [Formula: see text] as quintessence scalar field we have explored the possibility of quasi-exponential expansion and warm inflation.

Inhomogeneous early viscous fluid universe: A concrete model for dark energy

2016 ◽  
Vol 13 (04) ◽  
pp. 1650037
Author(s):  
G. S. Khadekar
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Bulk Viscosity ◽  
Scale Factor ◽  
Inhomogeneous Equation ◽  
Special Choice ◽  
Field Equations ◽  
Dependent Parameter ◽  
Frw Model ◽  
The Universe

In this paper the dynamical equation of the scale factor of the universe is investigated to describe the effect of bulk viscosity on the early evolution of the universe. We assume the inhomogeneous equation of state of the form [Formula: see text], where the adiabatic parameter [Formula: see text] varies with the scale factor [Formula: see text] proposed by Carvalho [Unified description of early universe, Int. J. Theor. Phys. 35 (1996) 2019] and [Formula: see text] is a time-dependent parameter in the framework of the flat FRW model. From this modified equation of state the exact solution of the field equations is obtained by considering the bulk viscosity is a linear combination of two terms of the form: [Formula: see text] and cosmological constant [Formula: see text], where [Formula: see text] and [Formula: see text] are constants, in which an inflationary phase is followed by the radiation dominated phase. For a special choice of the parameter we can explain the dark energy dominant universe and Friedmann equations are solved for two different phases of the universe and obtain the [Formula: see text]–[Formula: see text] relation.

scholarly journals Dynamical system analysis of FLRW models with Modified Chaplygin gas

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Ali Osman Yılmaz ◽  
Ertan Güdekli
Keyword(s):  
Dynamical System ◽  
Equation Of State ◽  
Energy Density ◽  
System Analysis ◽  
Chaplygin Gas ◽  
Modified Chaplygin Gas ◽  
State Spaces ◽  
The Universe ◽  
Matter Energy ◽  
Far Future

AbstractWe investigate Friedmann–Lamaitre–Robertson–Walker (FLRW) models with modified Chaplygin gas and cosmological constant, using dynamical system methods. We assume $$p=(\gamma -1)\mu -\dfrac{A}{\mu ^\alpha }$$ p = ( γ - 1 ) μ - A μ α as equation of state where $$\mu$$ μ is the matter-energy density, p is the pressure, $$\alpha$$ α is a parameter which can take on values $$0<\alpha \le 1$$ 0 < α ≤ 1 as well as A and $$\gamma$$ γ are positive constants. We draw the state spaces and analyze the nature of the singularity at the beginning, as well as the fate of the universe in the far future. In particular, we address the question whether there is a solution which is stable for all the cases.

scholarly journals Almost-Pseudo-Ricci Symmetric FRW Universe with a Dynamic Cosmological Term and Equation of State

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 205
Author(s):  
Sanjay Mandal ◽  
Avik De ◽  
Tee-How Loo ◽  
Pradyumn Kumar Sahoo
Keyword(s):  
Equation Of State ◽  
Cosmological Parameters ◽  
Cosmological Term ◽  
Ricci Scalar ◽  
Energy Conditions ◽  
Late Time ◽  
Frw Universe ◽  
Accelerating Expansion ◽  
Expansion Of The Universe ◽  
The Universe

The objective of the present paper is to investigate an almost-pseudo-Ricci symmetric FRW spacetime with a constant Ricci scalar in a dynamic cosmological term Λ(t) and equation of state (EoS) ω(t) scenario. Several cosmological parameters are calculated in this setting and thoroughly studied, which shows that the model satisfies the late-time accelerating expansion of the universe. We also examine all of the energy conditions to check our model’s self-stability.

FRW viscous fluid cosmological model with time-dependent inhomogeneous equation of state

2017 ◽  
Vol 15 (01) ◽  
pp. 1830001 ◽  
Author(s):  
G. S. Khadekar ◽  
Deepti Raut
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Quadratic Form ◽  
State Parameter ◽  
The Other ◽  
Time Dependent ◽  
Inhomogeneous Equation ◽  
Field Equations ◽  
Equation Of State Parameter ◽  
Viscous Models

In this paper, we present two viscous models of non-perfect fluid by avoiding the introduction of exotic dark energy. We consider the first model in terms of deceleration parameter [Formula: see text] has a viscosity of the form [Formula: see text] and the other model in quadratic form of [Formula: see text] of the type [Formula: see text]. In this framework we find the solutions of field equations by using inhomogeneous equation of state of form [Formula: see text] with equation of state parameter [Formula: see text] is constant and [Formula: see text].

scholarly journals Neutron stars as probes of dark matter

2020 ◽  
Vol 29 (14) ◽  
pp. 2043028
Author(s):  
M. Ángeles Pérez-García ◽  
Joseph Silk
Keyword(s):  
General Relativity ◽  
Dark Matter ◽  
Equation Of State ◽  
Neutron Stars ◽  
Internal Structure ◽  
Stellar Objects ◽  
Ordinary Matter ◽  
Stable Solutions ◽  
The Universe

Neutron Stars (NSs) are compact stellar objects that are stable solutions in General Relativity. Their internal structure is usually described using an equation of state that involves the presence of ordinary matter and its interactions. However there is now a large consensus that an elusive sector of matter in the universe, described as dark matter, remains as yet undiscovered. In such a case, NSs should contain both, baryonic and dark matter. We argue that depending on the nature of the dark matter and in certain circumstances, the two matter components would form a mixture inside NSs that could trigger further changes, some of them observable. The very existence of NSs constrains the nature and interactions of dark matter in the universe.

Dark energy, cosmic coincidence and future singularity of the universe

2012 ◽  
Vol 45 (1) ◽  
pp. 53-62 ◽  
Author(s):  
Sanjay Sarkar ◽  
Chandra Rekha Mahanta
Keyword(s):  
Dark Energy ◽  
Future Singularity ◽  
The Universe

scholarly journals A thermodynamic origin for the Cohen-Kaplan-Nelson bound

2021 ◽  
Author(s):  
Satish Ramakrishna
Keyword(s):  
General Relativity ◽  
Free Energy ◽  
Equation Of State ◽  
Density Of States ◽  
Canonical Ensemble ◽  
Field Theories ◽  
Quantum Field Theories ◽  
Quantum Field ◽  
Local Quantum ◽  
The Universe

Abstract The Cohen-Kaplan-Nelson bound is imposed on the grounds of logical consistency (with classical General Relativity) upon local quantum field theories. This paper puts the bound into the context of a thermodynamic principle applicable to a field with a particular equation of state in an expanding universe. This is achieved without overtly appealing to either a decreasing density of states or a minimum coupling requirement, though they might still be consistent with the results described. The paper establishes that the holographic principle applied to cosmology is consistent with minimizing the free energy of the universe in the canonical ensemble, upon the assumption that the ultraviolet cutoff is a function of the causal horizon scale.

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